1. Field of the Invention
The present invention relates to an electrodynamic speaker device that is installed in a vehicle cabin or the like, the amount of space therein being considerably limited, and in particular, to such a speaker device that is preferably used to reproduce sounds in a low-frequency range.
2. Description of the Related Art
For speaker devices that are preferably used to reproduce sounds in a low-frequency range, such as subwoofers, it is important to adjust the back pressure of a speaker unit mounted in a cabinet. Therefore, some of such speaker devices have a duct for adjusting the back pressure, which is formed so as to protrude from the cabinet. When the speaker unit emits a reproduced sound, air in the cabinet, which vibrates together with the diaphragm, passes through the duct. Therefore, the additional mass of the duct (air resistance in the duct) greatly influences the back pressure of the speaker unit. Accordingly, the back pressure can be adjusted so as to increase the pressure of sounds in an intended low-frequency range by appropriately setting the additional mass of the duct. For example, the back pressure increases when the additional mass is increased by increasing the length of the duct or by decreasing the cross-sectional area of the duct. Conversely, the back pressure decreases when the additional mass is decreased by decreasing the length of the duct or by increasing the cross-sectional area of the duct.
Some existing speaker devices include a cabinet in which a compartment for adjusting the back pressure is additionally formed (see, for example, Japanese Unexamined Patent Application Publication No. 2003-523673 corresponding to U.S. Pat. No. 6,389,146). In such speaker devices, the inside of the cabinet is divided into a plurality of compartments by partition walls, and a compartment in which a speaker unit is mounted and a vacant compartment communicate with each other through a duct formed in a partition wall. Therefore, the back pressure can be adjusted comparatively easily.
When installing a speaker device such as a subwoofer, which includes a cabinet and a duct for adjusting the back pressure protruding from the cabinet, in a vehicle cabin, it is preferable that the duct communicate with the outside of the vehicle cabin. Therefore, in some cases, the duct is connected to an opening formed in a partition wall that separates the inside of the vehicle cabin (acoustic space) from the outside of the vehicle cabin (outer space). Such an opening may be formed beforehand or may be additionally formed. However, because the amount of space in the vehicle cabin is limited, it may be difficult to dispose a cabinet having a duct at a position adjacent to the partition wall. In such a case, the length of the duct in the speaker device may be increased so that the duct can be connected to the opening in the partition wall. However, if the length of the duct is increased, the additional mass of the duct is increased, and the flow of air is excessively restrained when the diaphragm vibrates, and, as a result, the capability of the speaker device to reproduce low-frequency sounds is considerably impaired.
In the case of existing speaker device disclosed in Japanese Unexamined Patent Application Publication No. 2003-523673, in which the back pressure is adjusted by dividing the inside of the cabinet into a plurality of compartments, the size of the cabinet is necessarily increased. Therefore, in general, it is difficult to install such a speaker device in a space, such as a vehicle cabin, the amount of space therein being considerably limited.